High performance dielectric stack for dram capacitor

ABSTRACT

A method for fabricating a DRAM capacitor stack is described wherein the dielectric material is a multi-layer stack formed from a highly-doped material combined with a lightly or non-doped material. The highly-doped material remains amorphous with a crystalline content of less than 30% after an annealing step. The lightly or non-doped material becomes crystalline with a crystalline content of equal to or greater than 30% after an annealing step. The dielectric multi-layer stack maintains a high k-value while minimizing the leakage current and the EOT value.

This document relates to the subject matter of a joint researchagreement between Intermolecular, Inc. and Elpida Memory, Inc

FIELD OF THE INVENTION

The present invention generally relates to the field of dynamic randomaccess memory (DRAM), and more particularly to methods of forming acapacitor stack for improved DRAM performance.

BACKGROUND OF THE INVENTION

Dynamic Random Access Memory utilizes capacitors to store bits ofinformation within an integrated circuit. A capacitor is formed byplacing a dielectric material between two electrodes formed fromconductive materials. A capacitor's ability to hold electrical charge(i.e., capacitance) is a function of the surface area of the capacitorplates A, the distance between the capacitor plates d, and the relativedielectric constant or k-value of the dielectric material. Thecapacitance is given by:

$\begin{matrix}{C = {{\kappa ɛ}_{o}\frac{A}{d}}} & \left( {{Eqn}.\mspace{14mu} 1} \right)\end{matrix}$

where ∈_(o) represents the vacuum permittivity.

The dielectric constant is a measure of a material's polarizability.Therefore, the higher the dielectric constant of a material, the moreelectrical charge the capacitor can hold. Therefore, for a given desiredcapacitance, if the k-value of the dielectric is increased, the area ofthe capacitor can be decreased to maintain the same cell capacitance.Reducing the size of capacitors within the device is important for theminiaturization of integrated circuits. This allows the packing ofmillions (mega-bit (Mb)) or billions (giga-bit (Gb)) of memory cellsinto a single semiconductor device. The goal is to maintain a large cellcapacitance (generally ˜10 to 25 fF) and a low leakage current(generally <10⁻⁷ A cm⁻²). The physical thickness of the dielectriclayers in DRAM capacitors could not be reduced unlimitedly in order toavoid leakage current caused by tunneling mechanisms which exponentiallyincrease as the thickness of the dielectric layer decreases.

Traditionally, SiO₂ has been used as the dielectric material andsemiconducting materials (semiconductor-insulator-semiconductor [SIS]cell designs) have been used as the electrodes. The cell capacitance wasmaintained by increasing the area of the capacitor using very complexcapacitor morphologies while also decreasing the thickness of the SiO₂dielectric layer. Increases of the leakage current above the desiredspecifications have demanded the development of new capacitorgeometries, new electrode materials, and new dielectric materials. Celldesigns have migrated to metal-insulator-semiconductor (MIS) and now tometal-insulator-metal (MIM) cell designs for higher performance.

Typically, DRAM devices at technology nodes of 80 nm and below use MIMcapacitors wherein the electrode materials are metals. These electrodematerials generally have higher conductivities than the semiconductorelectrode materials, higher work functions, exhibit improved stabilityover the semiconductor electrode materials, and exhibit reduceddepletion effects. The electrode materials must have high conductivityto ensure fast device speeds. Representative examples of electrodematerials for MIM capacitors are metals, conductive metal oxides,conductive metal silicides, conductive metal nitrides (i.e. TiN), orcombinations thereof. MIM capacitors in these DRAM applications utilizeinsulating materials having a dielectric constant, or k-value,significantly higher than that of SiO₂ (k=3.9). For DRAM capacitors, thegoal is to utilize dielectric materials with k values greater than about40. Such materials are generally classified as high-k materials.Representative examples of high-k materials for MIM capacitors arenon-conducting metal oxides, non-conducting metal nitrides,non-conducting metal silicates or combinations thereof. Thesedielectrics may also include additional dopant materials.

A figure of merit in DRAM technology is the electrical performance ofthe dielectric material as compared to SiO₂ known as the EquivalentOxide Thickness (EOT). A high-k material's EOT is calculated using anormalized measure of silicon dioxide (SiO₂ k=3.9) as a reference, givenby:

$\begin{matrix}{{EOT} = {\frac{3.9}{\kappa} \cdot d}} & \left( {{Eqn}.\mspace{14mu} 2} \right)\end{matrix}$

where d represents the physical thickness of the capacitor dielectric.

As DRAM technologies scale below the 40 nm technology node,manufacturers must reduce the EOT of the high-k dielectric films in MIMcapacitors in order to increase charge storage capacity. The goal is toutilize dielectric materials that exhibit an EOT of less than about 0.8nm while maintaining a physical thickness of about 5-20 nm.

One class of high-k dielectric materials possessing the characteristicsrequired for implementation in advanced DRAM capacitors are high-k metaloxide materials. Titanium dioxide (TiO₂) and zirconium dioxide (ZrO₂)are two metal oxide dielectric materials which display significantpromise in terms of serving as high-k dielectric materials forimplementation in DRAM capacitors. Other metal oxide high-k dielectricmaterials that have attracted attention include HfO₂, Al₂O₃, Ta₂O₅, etc.

Generally, as the dielectric constant of a material increases, the bandgap of the material decreases. This leads to high leakage current in thedevice. As a result, without the utilization of countervailing measures,capacitor stacks implementing high-k dielectric materials may experiencelarge leakage currents. High work function electrodes (e.g., electrodeshaving a work function of greater than 5.0 eV) may be utilized in orderto counter the effects of implementing a reduced band gap high-kdielectric layer within the DRAM capacitor. Metals, such as platinum,gold, ruthenium, and ruthenium oxide are examples of high work functionelectrode materials suitable for inhibiting device leakage in a DRAMcapacitor having a high-k dielectric layer. The noble metal systems,however, are prohibitively expensive when employed in a mass productioncontext. Moreover, electrodes fabricated from noble metals often sufferfrom poor manufacturing qualities, such as surface roughness, pooradhesion, and form a contamination risk in the fab.

Leakage current in capacitor dielectric materials can be due to Schottkyemission, Frenkel-Poole defects (e.g. oxygen vacancies (V_(ox)) or grainboundaries), or Fowler-Nordheim tunneling. Schottky emission, alsocalled thermionic emission, is a common mechanism and is theheat-induced flow of charge over an energy barrier whereby the effectivebarrier height of a MIM capacitor controls leakage current. Theeffective barrier height is a function of the difference between thework function of the electrode and the electron affinity of thedielectric. The electron affinity of a dielectric is closely related tothe conduction band offset of the dielectric. The Schottky emissionbehavior of a dielectric layer is generally determined by the propertiesof the dielectric/electrode interface. Frenkel-Poole emission allows theconduction of charges through a dielectric layer through the interactionwith defect sites such as vacancies, grain boundaries, and the like. Assuch, the Frenkel-Poole emission behavior of a dielectric layer isgenerally determined by the dielectric layer's bulk properties.Fowler-Nordheim emission allows the conduction of charges through adielectric layer through tunneling. As such, the Fowler-Nordheimemission behavior of a dielectric layer is generally determined by thephysical thickness of the dielectric layer. This leakage current is aprimary driving force in the adoption of high-k dielectric materials.The use of high-k materials allows the physical thickness of thedielectric layer to be as thick as possible while maintaining therequired capacitance (see Eqn 1 above).

Additionally, DRAM capacitor stacks may undergo various refinementprocess steps after fabrication. These refinement processes may includepost-fabrication chemical and thermal processing (i.e., oxidation orreduction). For instance, after initial DRAM capacitor stackfabrication, a number of high temperature (up to about 60° C.) processesmay be applied to complete the device fabrication. During thesesubsequent process steps, the DRAM capacitor materials must remainchemically, physically, and structurally stable. They must maintain thestructural, compositional, physical, and electrical properties that havebeen developed. Furthermore, they should not undergo significantinteraction or reaction which may degrade the performance of the DRAMcapacitor.

Therefore, there is a need to develop methods to fabricate DRAMcapacitor stacks that exhibit a high capacitance due to the high k-valueof the capacitor dielectric, exhibit low leakage current, and exhibit alow EOT value.

SUMMARY OF THE DISCLOSURE

In some embodiments of the present invention, a multi-layer dielectricstack is formed from a highly doped material and a lightly or non-dopedmaterial. The highly doped material will remain amorphous (<30%crystalline) after an anneal step. The lightly or non-doped materialwill become crystalline (30% crystalline) after an anneal step. In someembodiments, the multi-layer dielectric stack is formed from ZrO₂. Insome embodiments, the highly doped ZrO₂ material is formed first on thebottom electrode and the lightly or non-doped ZrO₂ material is formed ontop of the highly doped material. In some embodiments, the lightly ornon-doped ZrO₂ material is formed first on the bottom electrode and thehighly doped ZrO₂ material is formed on top of the lightly or non-dopedmaterial. In some embodiments, the multi-layer dielectric stack isformed from TiO₂. In some embodiments, the lightly or non-doped TiO₂material is formed first on the bottom electrode and the highly dopedTiO₂ material is formed on top of the lightly or non-doped material.

BRIEF DESCRIPTION OF THE DRAWINGS

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. The drawings are not to scale and the relative dimensionsof various elements in the drawings are depicted schematically and notnecessarily to scale.

The techniques of the present invention can readily be understood byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

FIG. 1 is a flow chart illustrating a method for fabricating a DRAMcapacitor stack in accordance with some embodiments of the presentinvention.

FIG. 2 is a flow chart illustrating a method for fabricating a DRAMcapacitor stack in accordance with some embodiments of the presentinvention.

FIG. 3 illustrates a simplified cross-sectional view of a DRAM capacitorstack fabricated in accordance with some embodiments of the presentinvention.

FIG. 4 illustrates a simplified cross-sectional view of a DRAM capacitorstack fabricated in accordance with some embodiments of the presentinvention.

FIG. 5 illustrates a simplified cross-sectional view of a DRAM capacitorstack fabricated in accordance with some embodiments of the presentinvention.

FIG. 6 illustrates a simplified cross-sectional view of a DRAM memorycell fabricated in accordance with some embodiments of the presentinvention.

DETAILED DESCRIPTION

A detailed description of one or more embodiments is provided belowalong with accompanying figures. The detailed description is provided inconnection with such embodiments, but is not limited to any particularexample. The scope is limited only by the claims and numerousalternatives, modifications, and equivalents are encompassed. Numerousspecific details are set forth in the following description in order toprovide a thorough understanding. These details are provided for thepurpose of example and the described techniques may be practicedaccording to the claims without some or all of these specific details.For the purpose of clarity, technical material that is known in thetechnical fields related to the embodiments has not been described indetail to avoid unnecessarily obscuring the description.

The dielectric constant of a dielectric material is dependent upon thecrystalline phase(s) of the material. For example, in the case of TiO₂,the anatase crystalline phase of TiO₂ has a dielectric constant ofapproximately 40, while the rutile crystalline phase of TiO₂ can have adielectric constant of approximately >80. Due to the higher-k value ofthe rutile-phase, it is desirable to produce TiO₂ based DRAM capacitorswith the TiO₂ in the rutile-phase. Similarly, the cubic and tetragonalphases of ZrO₂ have a higher-k value than the monoclinic phase. So, itis desirable to produce ZrO₂ based DRAM capacitors with the ZrO₂ in thecubic or tetragonal phase.

The crystal phase of an underlying layer can be used to influence thegrowth of a specific crystal phase of a subsequent material if theircrystal structures are similar and their lattice constants are similar.This technique is well known in technologies such as epitaxial growth.The same concepts have been extended to the growth of thin films wherethe underlying layer can be used as a “template” to encourage the growthof a desired phase over other competing crystal phases.

Conductive metal oxides, conductive metal silicides, conductive metalcarbides, conductive metal nitrides, or combinations thereof compriseother classes of materials that may be suitable as DRAM capacitorelectrodes. Generally, transition metals and their conductive binarycompounds form good candidates as electrode materials. The transitionmetals exist in several oxidation states. Therefore, a wide variety ofcompounds are possible. Different compounds may have different crystalstructures, electrical properties, etc. It is important to utilize theproper compound for the desired application. Conductive metal nitridessuch as TiN, TaN, WN, etc. have attracted interest as DRAM capacitorelectrodes with TiN being the most popular.

In one example of conductive metal oxides, molybdenum has several oxidephases of which MoO₂ and MoO₃ are two examples. These two oxides ofmolybdenum have different properties. MoO₂ is conductive and has showngreat promise as an electrode material in DRAM capacitors. MoO₂ has adistorted rutile crystal structure and serves as an acceptable templateto promote the deposition of the rutile-phase of TiO₂ as discussedabove. MoO₂ also has a high work function (can be >5.0 eV depending onprocess history) which helps to minimize the leakage current of the DRAMdevice. However, oxygen-rich phases (MoO_(2+x)) degrade the performanceof the MoO₂ electrode because they do not promote the deposition of therutile-phase of TiO₂. For example, MoO₃ (the most oxygen-rich phase) isan insulator and has an orthorhombic crystal structure.

Generally, a deposited thin film may be amorphous, crystalline, or amixture thereof. Furthermore, several different crystalline phases mayexist. Therefore, processes (both deposition and post-treatment) must bedeveloped to maximize the formation of the desired composition andcrystalline phase of the thin film. The thin films used to form the MIMDRAM capacitor stack may be formed using any common formation techniquesuch as atomic layer deposition (ALD), plasma enhanced atomic layerdeposition (PE-ALD), atomic vapor deposition (AVD), ultraviolet assistedatomic layer deposition (UV-ALD), chemical vapor deposition (CVD),plasma enhanced chemical vapor deposition (PECVD), or physical vapordeposition (PVD).

The crystalline phases of dielectric materials exhibit higher-k valuesthan the amorphous phases. Therefore, there is often an optional annealstep either after the dielectric formation step (also known as a postdielectric anneal (PDA)) or an anneal step after the formation of thetop electrode (also known as a post metallization anneal (PMA)) tocrystallize the dielectric layer. Examples of the PDA and PMA treatmentsare further described in U.S. application Ser. No. 13/159,842 filed onJun. 14, 2011, entitled “METHOD OF PROCESSING MIM CAPACITORS TO REDUCELEAKAGE CURRENT” and having internal docket number (IM0223_US) and isincorporated herein by reference.

Dopants can be added to the dielectric material to increase the k-valueand/or decrease the leakage current. As used herein, the dopant may beelectrically active or not electrically active. The definition excludesresidues and impurities such as carbon, etc. that may be present in thematerial due to inefficiencies of the process or impurities in theprecursor materials. The concentration of the dopant is one factor thataffects the crystallinity of the dielectric material. Other factors thataffect the crystallinity of the dielectric material comprise annealingtime, annealing temperature, film thickness, etc. Generally, as theconcentration of the dopant is increased, the crystallizationtemperature of the dielectric material increases. As an example, TiO₂dielectric materials doped with Al at a concentration of less than about10 atomic % will crystallize when annealed at 550C for 10 minutes.However, if the Al concentration is increased to about 15 atomic % orabove, then the TiO₂ dielectric materials remain amorphous after beingannealed at 550C for 10 minutes. Other suitable dopants for use withTiO₂ dielectric materials comprise Zr, Ge, Hf, Sn, Sr, Y, Si, La, Er,Ga, Gd, Mg, Co, or combinations thereof. Dopants can also be added toZrO₂ dielectric materials. Suitable dopants for use with ZrO₂ dielectricmaterials comprise Al, Ge, Hf, Sn, Sr, Y, Si, Ti, La, Er, Ga, Gd, Mg,Co, or combinations thereof.

Lightly or non-doped dielectric materials wherein the dopantconcentration is low will generally be crystalline after being annealedat 550C for 5 minutes, will exhibit high k-values, but sometimes willalso exhibit higher leakage current. As used herein, a dielectricmaterial will be considered to be crystalline if it exhibits greaterthan or equal to 30% crystallinity as measured by a technique such asx-ray diffraction (XRD) after an anneal step. The higher leakage currentis partly due to leakage along the grain boundaries due to theFrenkel-Poole mechanism discussed earlier. Conversely, highly dopeddielectric materials wherein the dopant concentration is high willgenerally be amorphous after being annealed at 550C for 5 minutes, willexhibit lower k-values, but will also exhibit low leakage current.Typically, dielectric materials that are doped with dopant levels aboveabout 10 to 15 atomic % will remain amorphous after the typical annealsteps used in the fabrication of DRAM devices. However, this will varydepending on the dielectric material, the dopant, and the details of theannealing step and is not intended to be limiting. The properties ofthese two types of dielectric materials may be combined to concurrentlymeet the EOT (a function of the k-value) and the leakage current targetsfor advanced DRAM technologies.

FIG. 1 describes a method, 100, for fabricating a DRAM capacitor stack.The initial step, 102, comprises forming a first electrode layer.Examples of suitable electrode materials comprise metals, conductivemetal oxides, conductive metal silicides, conductive metal carbides,conductive metal nitrides, or combinations thereof. Two particularlyinteresting classes of materials are the conductive metal nitrides andthe conductive metal oxides. The first electrode layer may receive anoptional treatment (not shown). In the next step, 104, a multi-layerdielectric material is formed wherein one of the layers comprises alightly doped material, and the other layer comprises a highly dopedmaterial. After a later annealing step (either PDA or PMA, not shown),the lightly or non-doped material will become at least partiallycrystallized (30% crystalline). These materials will also generallyexhibit a higher k value, but generally exhibit higher leakage current.After the later annealing step, the highly doped material will remainsubstantially amorphous (<30% crystalline). The crystallinity may bedetermined by a technique such as XRD. These materials will alsogenerally exhibit a lower k value, but generally exhibit lower leakagecurrent. In the next step, 106, a second electrode layer is formed onthe multi-layer dielectric material. The capacitor stack may receive anoptional PMA anneal (not shown).

FIG. 2 illustrates a flow chart of the formation of a capacitor stackaccording to some embodiments of the present invention. In step 202, afirst electrode layer is formed. Examples of suitable electrodematerials comprise metals, conductive metal oxides, conductive metalsilicides, conductive metal carbides, conductive metal nitrides, orcombinations thereof. Step 204 is an optional step wherein the firstelectrode layer may undergo a treatment before the dielectric materialis formed. If the first electrode layer is a conductive metal nitridematerial, then the first electrode layer may be annealed using a RapidThermal Anneal (RTA) technique or furnace anneal technique. For RTAcase, the temperature is quickly raised in the presence of a nitrogencontaining gas such as N₂, forming gas, NH₃, etc. Examples of suchelectrode treatment steps are further described in U.S. application Ser.No. 13/051,531 filed on Mar 18, 2011, which is incorporated herein byreference. Alternatively, if the first electrode is a conductive metaloxide, then the first electrode layer may be annealed in an inert orreducing atmosphere such as Ar, N₂, or forming gas. Examples of such anannealing process is further described in U.S. application Ser. No.13/084,666 filed on Apr. 12, 2011, entitled “METHOD FOR FABRICATING ADRAM CAPACITOR” and is incorporated herein by reference.

The method described in the flow chart can now take one of two possiblepaths. In step 206, a multi-layer dielectric material is formed whereinthe first material is formed from a lightly or non-doped material. Thematerial may be deposited with crystalline phases present or it may bedeposited as an amorphous material and crystallized during a later step.A second material of the dielectric material is then formed on top ofthe first material. The second material is formed from a highly dopedmaterial. The second material may be the same bulk dielectric materialor may be a different bulk dielectric material as the first material. Inpractice, the second material is advantageously the same bulk dielectricmaterial as the first material. Additionally, the dopant used in thesecond material may be the same dopant or may be a different dopant asthat used in the first material. In practice, the dopant used in thesecond material is advantageously the same as the dopant used the firstmaterial.

In step 208, an alternative method is described wherein a multi-layerdielectric material is formed wherein the first material is formed froma highly doped material. A second material of the dielectric material isthen formed on top of the first material. The second material is formedfrom a lightly or non-doped material. The material may be deposited withcrystalline phases present or it may be deposited as an amorphousmaterial and crystallized during a later step. The second material maybe the same bulk dielectric material or may be a different bulkdielectric material as the first material. In practice, the secondmaterial is advantageously the same bulk dielectric material as thefirst material. Additionally, the dopant used in the second material maybe the same dopant or may be a different dopant as that used in thefirst material. In practice, the dopant used in the second material isadvantageously the same as the dopant used the first material.

After the dielectric material has been formed, an optional PDA treatmentmay be performed as shown in step, 210 as discussed earlier and in theincorporated reference. In step, 212, a second electrode layer is formedon top of the multi-layer dielectric material. Examples of suitableelectrode materials comprise metals, conductive metal oxides, conductivemetal silicides, conductive metal carbides, conductive metal nitrides,or combinations thereof. After the second electrode has been formed, anoptional PMA treatment may be performed as shown in step, 214 asdiscussed earlier and in the incorporated reference.

FIG. 3 illustrates a simplified cross-sectional view of a DRAM capacitorstack fabricated in accordance with some embodiments of the presentinvention. This example will use ZrO₂ as the dielectric material.However, those skilled in the art will understand that the presentmethods may be applied to many dielectric materials. First electrode,302, is formed on substrate, 201. When ZrO₂ is used as the dielectricmaterial, the first electrode is advantageously TiN. The TiN firstelectrode may optionally receive a treatment before the formation of themulti-layer dielectric material as discussed previously. Lightly ornon-doped ZrO₂ material, 304, is then formed on top of the TiN firstelectrode. Suitable dopants for use with ZrO₂ dielectric materialscomprise Al, Ge, Hf, Sn, Sr, Y, Si, Ti, La, Er, Ga, Gd, Mg, Co, orcombinations thereof. Highly doped ZrO₂ material, 306, is then formed ontop of the lightly doped ZrO₂ material. Suitable dopants for use withZrO₂ dielectric materials comprise Al, Ge, Hf, Sn, Sr, Y, Si, Ti, La,Er, Ga, Gd, Mg, Co, or combinations thereof. The multi-layer dielectricmaterial may optionally receive a PDA treatment as discussed previously.Second electrode, 308, is then formed on top of the multi-layerdielectric material. When ZrO₂ is used as the dielectric material, thesecond electrode is advantageously TiN. The capacitor stack mayoptionally receive a PMA treatment as discussed previously.

FIG. 4 illustrates a simplified cross-sectional view of a DRAM capacitorstack fabricated in accordance with some embodiments of the presentinvention. This example will use ZrO₂ as the dielectric material.However, those skilled in the art will understand that the presentmethods may be applied to many dielectric materials. First electrode,402, is formed on substrate, 201. When ZrO₂ is used as the dielectricmaterial, the first electrode is advantageously TiN. The TiN firstelectrode may optionally receive a treatment before the formation of themulti-layer dielectric material as discussed previously. Highly dopedZrO₂ material, 404, is then formed on top of the TiN first electrode.Suitable dopants for use with ZrO₂ dielectric materials comprise Al, Ge,Hf, Sn, Sr, Y, Si, Ti, La, Er, Ga, Gd, Mg, Co, or combinations thereof.Lightly or non-doped ZrO₂ material, 406, is then formed on top of thehighly doped amorphous material. Suitable dopants for use with ZrO₂dielectric materials comprise Al, Ge, Hf, Sn, Sr, Y, Si, Ti, La, Er, Ga,Gd, Mg, Co, or combinations thereof. The multi-layer dielectric materialmay optionally receive a PDA treatment as discussed previously. Secondelectrode, 408, is then formed on top of the multi-layer dielectricmaterial. When ZrO₂ is used as the dielectric material, the secondelectrode is advantageously TiN. The capacitor stack may optionallyreceive a PMA treatment as discussed previously.

FIG. 5 illustrates a simplified cross-sectional view of a DRAM capacitorstack fabricated in accordance with some embodiments of the presentinvention. This example will use TiO₂ as the dielectric material.However, those skilled in the art will understand that the presentmethods may be applied to many dielectric materials. First electrode,502, is formed on substrate, 201. When TiO₂ is used as the dielectricmaterial, the first electrode is advantageously a conductive metal oxidethat can act as a template for the rutile phase of TiO₂. Advantageously,the first electrode is crystalline MoO₂ when the dielectric material isTiO₂. The MoO₂ first electrode may optionally receive an annealtreatment before the formation of the multi-layer dielectric material asdiscussed previously. Lightly doped TiO₂ material, 504, is then formedon top of the crystalline MoO₂ first electrode. Suitable dopants for usewith TiO₂ dielectric materials comprise Al, Zr, Ge, Hf, Sn, Sr, Y, Si,La, Er, Ga, Gd, Mg, Co, or combinations thereof. When TiO₂ is used asthe dielectric material, the lightly doped material must be on thebottom so that the high k rutile phase of TiO₂ is formed on thecrystalline MoO₂ first electrode. If the highly doped material is formedon the first electrode, the multi-layer dielectric material does notexhibit a high k-value due to the absence of a large fraction of rutilephase TiO₂. Highly doped TiO₂ material, 506, is then formed on top ofthe lightly doped TiO₂ material. Suitable dopants for use with TiO₂dielectric materials comprise Al, Zr, Ge, Hf, Sn, Sr, Y, Si, La, Er, Ga,Gd, Mg, Co, or combinations thereof. The multi-layer dielectric materialmay optionally receive a PDA treatment as discussed previously. Secondelectrode, 508, is then formed on top of the multi-layer dielectricmaterial. When TiO₂ is used as the dielectric material, the secondelectrode is advantageously a metal or a conductive metal oxide. Thecapacitor stack may optionally receive a PMA treatment as discussedpreviously.

In the examples illustrated above, the multi-layer dielectric materialtypically is subjected to an anneal step either as a PDA treatment, aPMA treatment, or both. This anneal step crystallizes the lightly ornon-doped materials to yield a material with a high k-value. This allowsthe physical thickness of the multi-layer dielectric material to bethick enough to suppress the leakage current from Fowler-Nordheimtunneling mechanisms as discussed previously. The high concentration ofdoping in the highly doped material ensures that this material remainssubstantially amorphous under the anneal conditions. The lack of orsmall existence of grain boundaries in this material suppresses theleakage current from Frenkel-Poole mechanisms as discussed previously.As the amorphous material has a lower K value than the crystallinematerial, the thickness of the highly doped material is held to lessthan 25% of the total dielectric thickness to mitigate the adverseimpact on the EOT of the multi-layer dielectric stack.

An example of a specific application of some embodiments of the presentinvention is in the fabrication of capacitors used in the memory cellsin DRAM devices. DRAM memory cells effectively use a capacitor to storecharge for a period of time, with the charge being electronically “read”to determine whether a logical “one” or “zero” has been stored in theassociated cell. Conventionally, a cell transistor is used to access thecell. The cell transistor is turned “on” in order to store data on eachassociated capacitor and is otherwise turned “off” to isolate thecapacitor and preserve its charge. More complex DRAM cell structuresexist, but this basic DRAM structure will be used for illustrating theapplication of this disclosure to capacitor manufacturing and to DRAMmanufacturing. FIG. 6 is used to illustrate one DRAM cell, 620,manufactured using a bilayer dielectric structure as discussedpreviously. The cell, 620, is illustrated schematically to include twoprinciple components, a cell capacitor, 600, and a cell transistor, 602.The cell transistor is usually constituted by a MOS transistor having agate, 616, source, 612, and drain, 614. The gate is usually connected toa word line and one of the source or drain is connected to a bit line.The cell capacitor has a lower or storage electrode and an upper orplate electrode. The storage electrode is connected to the other of thesource or drain and the plate electrode is connected to a referencepotential conductor. The cell transistor is, when selected, turned “on”by an active level of the word line to read or write data from or intothe cell capacitor via the bit line.

As was described previously, the cell capacitor, 600, comprises a firstelectrode, 604, formed on substrate, 601. The first electrode, 604, isconnected to the source or drain of the cell transistor, 602. Forillustrative purposes, the first electrode has been connected to thesource, 612, in this example. As discussed previously, first electrode,604, may be subjected to an anneal in a reducing atmosphere before theformation of the dielectric layer if the first electrode is a conductivemetal oxide such as MoO₂ or the first electrode may receive othertreatments if the first electrode is a conductive metal nitride such asTiN. First dielectric layer, 606, is formed on top of the firstelectrode. As discussed previously, the first dielectric layer may bedoped. If the first dielectric layer is TiO₂, then the first dielectriclayer will be lightly or non-doped so that the rutile phase of TiO₂ canbe formed on the bottom electrode. If the first dielectric layer isanother high k material (i.e. ZrO₂, etc.), then the first dielectriclayer may be either lightly/non-doped or highly doped. Second dielectriclayer, 608, is formed on top of the first dielectric layer. As discussedpreviously, the second dielectric layer may be doped. If the seconddielectric layer is TiO₂, then the second dielectric layer will behighly doped so that it will remain amorphous (<30% crystalline) aftersubsequent anneal steps. If the second dielectric layer is another highk material (i.e. ZrO₂, etc.), then the second dielectric layer may beeither lightly or non-doped or highly doped. The second dielectric layerwill have a different doping level than the first dielectric layer.Typically, the bilayer dielectric material is then subjected to a PDAtreatment. The second electrode layer, 610, is then formed on top of thebilayer dielectric material. This completes the formation of thecapacitor stack. Typically, the capacitor stack is then subjected to aPMA treatment.

One specific example of the DRAM device described above consists of acapacitor stack comprising a MoO₂ bottom electrode layer, a lightly ornon-doped TiO₂ first dielectric layer, a highly doped TiO₂ seconddielectric layer, and a second electrode layer. A second example of theDRAM device described above consists of a capacitor stack comprising aTiN bottom electrode layer, a lightly or non-doped ZrO₂ first dielectriclayer, a highly doped ZrO₂ second dielectric layer, and a secondelectrode layer. A third example of the DRAM device described aboveconsists of a capacitor stack comprising a TiN bottom electrode layer, ahighly doped ZrO₂ first dielectric layer, a lightly or non-doped dopedZrO₂ second dielectric layer, and a second electrode layer.

Although the foregoing examples have been described in some detail forpurposes of clarity of understanding, the invention is not limited tothe details provided. There are many alternative ways of implementingthe invention. The disclosed examples are illustrative and notrestrictive.

1. A method for forming a capacitor stack comprising: forming a firstelectrode layer on a substrate; forming a first dielectric material onthe first electrode layer wherein the first dielectric material furthercontains a dopant; forming a second dielectric material on the firstdielectric material wherein the second dielectric material furthercontains a dopant and the doping level is greater than the dopant levelof the first dielectric material; forming a second electrode layer onthe second dielectric material; and performing an annealing step whereinthe first dielectric material is equal to or greater than 30%crystalline after the annealing step and the second dielectric materialis less than 30% crystalline after the annealing step.
 2. The method ofclaim 1 further comprising a post metallization anneal treatment afterthe forming of the second electrode layer.
 3. The method of claim 1wherein the first electrode layer is one of a metal, conductive metaloxide, conductive metal silicide, conductive metal carbides, conductivemetal nitride, or combinations thereof.
 4. The method of claim 1 whereinthe dopant comprises one of Al, Zr, Ge, Hf, Sn, Sr, Y, Si, Ti, La, Er,Ga, Gd, Mg, Co, or combinations thereof.
 5. The method of claim 1wherein the first electrode layer is TiN, the first dielectric materialis doped ZrO₂, the second dielectric material is doped ZrO₂, and thesecond electrode layer is TiN.
 6. The method of claim 1 wherein thefirst electrode layer is crystalline MoO₂, the first dielectric materialis doped TiO₂, and the second dielectric material is doped TiO₂.
 7. Themethod of claim 1 wherein the first dielectric material and the seconddielectric material are formed from the same bulk dielectric material.8. The method of claim 1 wherein the first dielectric material and thesecond dielectric material are formed from different bulk dielectricmaterials.
 9. The method of claim 1 wherein the dopant in the firstdielectric material and the dopant in the second dielectric material arethe same.
 10. The method of claim 1 wherein the dopant in the firstdielectric material and the dopant in the second dielectric material aredifferent.
 11. A method for forming a capacitor stack comprising:forming a first electrode layer on a substrate; forming a firstdielectric material on the first electrode layer wherein the firstdielectric material further contains a dopant; forming a seconddielectric material on the first dielectric material wherein the seconddielectric material further contains a dopant and the doping level isless than the doping level of the first dielectric material; forming asecond electrode layer on the second dielectric material; and performingan annealing step wherein the first dielectric material is less than 30%crystalline after the annealing step and the second dielectric materialis equal to or greater than 30% crystalline after the annealing step.12. The method of claim 11 further comprising a post metallizationanneal treatment after the forming of the second electrode layer. 13.The method of claim 11 wherein the first electrode layer is one of ametal, conductive metal oxide, conductive metal silicide, conductivemetal carbides, conductive metal nitride, or combinations thereof. 14.The method of claim 11 wherein the dopant comprises one of Al, Zr, Ge,Hf, Sn, Sr, Y, Si, Ti, La, Er, Ga, Gd, Mg, Co, or combinations thereof.15. The method of claim 11 wherein the first electrode layer is TiN, thefirst dielectric material is doped ZrO₂, the second dielectric materialis doped ZrO₂, and the second electrode is TiN.
 16. The method of claim11 wherein the first dielectric material and the second dielectricmaterial are formed from the same bulk dielectric material.
 17. Themethod of claim 11 wherein the first dielectric material and the seconddielectric material are formed from different bulk dielectric materials.18. The method of claim 11 wherein the dopant in the first dielectricmaterial and the dopant in the second dielectric material are the same.19. The method of claim 11 wherein the dopant in the first dielectricmaterial and the dopant in the second dielectric material are different.